Overwrite wp_enqueue_script under certain condition

The first wp_enqueue_script() in your code should be wp_register_script(). The enqueue function does the job while the register function is getting ready to do the job.

It is always better to rather create a new js file to keep things organised. As to calling them, you can do the following

wp_register_script( 'abc', 'path_to' );
wp_register_script( 'xyz', 'path_to' );

if ( 'condition_a' ) {
    wp_enqueue_script( 'abc' );
} else {
    wp_enqueue_script( 'xyz' );
}

As to whether or not to use the _register_ function, you can use it if you want to. When enqueueing scripts conditionaly, like above, you can use the register function to get the scripts ready for loading if they are called upon by the condition, BUT, it is not necesarry

EDIT

You can also use wp_localize_script() to pass PHP variables according to a certain condition to a js script, and then in your js script you can alter your script according to the value of the PHP condition